Derivative Financial Instruments (Notional and Fair Values of Derivative Financial Instruments) (Details) - USD ($) $ in Millions	Jun. 30, 2023	Dec. 31, 2022
Derivative [Line Items]
NOTIONAL VALUE	$ 323,910.1	$ 310,684.3
FAIR VALUE, ASSET	2,141.5	3,151.2
Total Gross Derivatives, asset	2,578.0	3,732.4
Less: Netting	1,245.9	2,810.7
Derivative Assets	1,332.1	921.7
FAIR VALUE, LIABILITY	2,076.7	2,548.9
Derivative Liabilities	2,607.1	3,654.1
Less: Netting	1,461.0	1,865.9
Derivative Liabilities	1,146.1	1,788.2
Derivatives Designated as Hedging under GAAP
Derivative [Line Items]
NOTIONAL VALUE	8,933.5	8,537.9
FAIR VALUE, ASSET	95.5	349.4
FAIR VALUE, LIABILITY	51.0	51.1
Fair Value Hedges | Derivatives Designated as Hedging under GAAP | Interest Rate Contracts
Derivative [Line Items]
NOTIONAL VALUE	4,323.1	4,622.0
FAIR VALUE, ASSET	57.0	58.5
FAIR VALUE, LIABILITY	26.5	32.7
Fair Value Hedges | Derivatives Not Designated as Hedging under GAAP
Derivative [Line Items]
NOTIONAL VALUE	314,976.6	302,146.4
FAIR VALUE, ASSET	2,482.5	3,383.0
FAIR VALUE, LIABILITY	2,556.1	3,603.0
Fair Value Hedges | Non-Designated Risk Management Derivatives
Derivative [Line Items]
NOTIONAL VALUE	888.8	773.6
FAIR VALUE, ASSET	0.2	0.4
FAIR VALUE, LIABILITY	36.7	34.9
Fair Value Hedges | Non-Designated Risk Management Derivatives | Foreign Exchange Contracts
Derivative [Line Items]
NOTIONAL VALUE	95.8	55.9
FAIR VALUE, ASSET	0.2	0.1
FAIR VALUE, LIABILITY	0.3	0.1
Fair Value Hedges | Non-Designated Risk Management Derivatives | Other Financial Derivatives
Derivative [Line Items]
NOTIONAL VALUE	793.0	717.7
FAIR VALUE, ASSET	0.0	0.3
FAIR VALUE, LIABILITY	36.4	34.8
Fair Value Hedges | Client-Related and Trading Derivatives
Derivative [Line Items]
NOTIONAL VALUE	314,087.8	301,372.8
FAIR VALUE, ASSET	2,482.3	3,382.6
FAIR VALUE, LIABILITY	2,519.4	3,568.1
Fair Value Hedges | Client-Related and Trading Derivatives | Foreign Exchange Contracts
Derivative [Line Items]
NOTIONAL VALUE	299,322.9	288,994.6
FAIR VALUE, ASSET	2,225.4	3,219.1
FAIR VALUE, LIABILITY	2,137.8	3,169.0
Fair Value Hedges | Client-Related and Trading Derivatives | Interest Rate Contracts
Derivative [Line Items]
NOTIONAL VALUE	14,764.9	12,378.2
FAIR VALUE, ASSET	256.9	163.5
FAIR VALUE, LIABILITY	381.6	399.1
Cash Flow Hedges | Derivatives Designated as Hedging under GAAP | Foreign Exchange Contracts
Derivative [Line Items]
NOTIONAL VALUE	750.0	150.9
FAIR VALUE, ASSET	19.2	7.4
FAIR VALUE, LIABILITY	0.9	5.7
Net Investment Hedges | Derivatives Designated as Hedging under GAAP | Foreign Exchange Contracts
Derivative [Line Items]
NOTIONAL VALUE	3,860.4	3,765.0
FAIR VALUE, ASSET	19.3	283.5
FAIR VALUE, LIABILITY	$ 23.6	$ 12.7